In network topology, a cable modem is a network bridge that conforms to IEEE 802.1D for Ethernet networking (with some modifications). The cable modem bridges Ethernet frames between a customer LAN and the coax cable network.

A cable modem does support functionalities at other layers. In physical layer (or layer 1), the cable modem supports the Ethernet PHY on its LAN interface, and a DOCSIS defined cable-specific PHY on its HFC cable interface. It is to this cable-specific PHY that the name cable modem refers. In the network layer (or layer 3), the cable modem is a IP host in that it has its own IP address used by the network operator to manage and troubleshoot the device. In the transport layer (or layer 4) the cable modem supports UDP in association with its own IP address, and it supports filtering based on TCP and UDP port numbers to, for example, block forwarding of NetBIOS traffic out of the customer's LAN. In the application layer (layer 5 or layer 7), the cable modem supports certain protocols that are used for management and maintenance, notably DHCP, SNMP, and TFTP.

Some cable modem devices may incorporate a router along with the cable modem functionality, to provide the LAN with its own IP network addressing. From a data forwarding and network topology perspective, this router functionality is typically kept distinct from the cable modem functionality (at least logically) even though the two may share a single enclosure and appear as one unit. So, the cable modem function will have its own IP address and MAC address as will the router.

History

Hybrid Networks

Hybrid Networks developed, demonstrated and patented the first high speed, asymmetrical cable modem systems in 1990. A key Hybrid Networks insight was that highly asymmetrical communications would be sufficient to satisfy consumers connected remotely to an otherwise completely symmetric high speed data communications network. This was important because it was very expensive to provide high speed in the upstream direction, while the CATV systems already had substantial broadband capacity in the downstream direction. Also key was that it saw that the upstream and downstream communications could be on the same or different communications media using different protocols working in each direction to establish a closed loop communications system. The speeds and protocols used in each direction would be very different. The earliest systems used the public switched telephone network (PSTN) for the return path since very few cable systems were bi-directional. Later systems used cable for the upstream as well as the downstream path.
There was extreme skepticism to this approach initially. In fact, many technical people doubted that it could work at all. Hybrid's system architecture is the way most cable modem systems operate today.

LANcity

LANcity was an early pioneer in cable modems, developing a proprietary system that saw fairly wide deployment in the US. LANcity was sold to Bay Networks which was then acquired by Nortel, which eventually spun the cable modem business off as ARRIS. ARRIS continues to make cable modems and CMTS equipment compliant with the DOCSIS standard.

Com21

Com21 was another early pioneer in cable modems, and quite successful until proprietary systems became obsolete when DOCSIS became the widely accepted standard. The Com21 system used a ComController as central bridge in CATV network head-ends, the ComPort cable modem in various models and the NMAPS management system using HP OpenView as platform. Later they also introduced a return path multiplexer to overcome noise problems when combining return path signals from multiple areas. The proprietary protocol was based on the ATM protocol. The central ComController switch was a modular system offering one downstream channel (transmitter) and one management module. The remaining slots could be used for upstream receivers (2 per card), dual ethernet 10BaseT and later also Fastethernet and ATM interfaces. The ATM interface became the most popular as it supported the increasing bandwidth demands and also supported VLAN's.
Com21 developed a DOCSIS modem, but didn't survive and filed bankruptcy in 2003.

CDLP

CDLP was a proprietary system that was made by Motorola. CDLP CPE was capable of both PSTN (telephone network) and RF (cable network) return paths. The PSTN return path cable modem service was considered 'one way cable' and had many of the same drawbacks as satellite Internet service, and as a result it quickly gave way to two way cable. Cable modems that used the RF cable network for the return path were considered 'two way cable', and were better able to compete with DSL which was bidirectional. The standard is more or less defunct now with new providers using, and existing providers having changed over to, the DOCSIS standard. The Motorola CDLP Proprietary CyberSURFR is an example of a modem that was built to the CDLP standard, capable of a peak 10 Mbit/s downstream and 1.532 Mbit/s upstream. (CDLP supported a maximum downstream bandwidth of 30 Mbit/s which could be reached by using several modems.)

The Australian ISP BigPond employed this system when it started cable modem trials in 1996. For a number of years cable Internet access was only available to Sydney, Melbourne and Brisbane via CDLP. This network ran parallel to the newer DOCSIS system for a number of years. In 2004 the CDLP network was switched off and now is exclusively DOCSIS.

IEEE 802.14

In the mid-1990s the IEEE 802 committee formed a subcommittee (802.14) to develop a standard for cable modem systems. While significant progress was made, the group was disbanded when North American MSOs instead backed the fledgling DOCSIS specification.

DOCSIS

In the late 1990s, a consortium of US cable operators, known as "MCNS" formed to quickly develop an open and interoperable cable modem specification. The group essentially combined technologies from the two dominant proprietary systems at the time, taking the physical layer from the MotorolaCDLP system and the MAC layer from the LANcity system. When the initial specification had been drafted, the MCNS consortium handed over control of it to CableLabs. CableLabs took on maintenance of the specification, promoted it in various standards organizations (notably SCTE and ITU), developed a certification testing program for cable modem equipment, and has since drafted multiple extensions to the original specification. Virtually all cable modems operating in the field today are compliant with one version or another of DOCSIS. Because of the differences in the European PAL and USA's NTSC systems two main versions of DOCSIS exist: DOCSIS and EuroDOCSIS. The main differences are found in the width of RF-channels: 6 MHz for the USA and 8 MHz for Europe. Nearly all current cablemodem systems use a version of this standard with the exception of Japan.

Cable modems and VoIP

With the advent of Voice over IP telephony, cable modems can also be used to provide telephone service. Many people who have cable modems have opted to eliminate their Plain Old Telephone Service (POTS). Because most telephone companies do not offer naked DSL (DSL service without a POTS line), VoIP use is higher amongst cable modem users.

A cable modem subscriber can make use of VoIP telephony by subscribing to a third party service (e.g. Vonage or Skype). As an alternative, many cable operators offer a VoIP service based on PacketCable. PacketCable allows MSOs to offer both High Speed Internet and VoIP through a single piece of customer premise equipment, known as an Embedded Multimedia Terminal Adapter (EMTA or E-MTA). An EMTA is basically a cable modem and a VoIP adapter (known as a Multimedia Terminal Adapter) bundled into a single device. PacketCable service has a significant technical advantage over third-party providers in that voice packets are given guaranteed Quality of Service across their entire path so that call quality can be assured.

To attain a higher quality VoIP telephone line, cable modem users often will take advantage of the two IP addresses that many ISPs provide. This type of setup allows the PCs to access the internet under one IP address while the VoIP gateway has its own IP address. This is advantageous to many VoIP users as it creates a network where their PC network activity doesn't affect their call quality. To achieve this setup, a switch/hub is placed downline of the modem. Doing so will assign two IP addresses (one to the computer network i.e. router and one to the voice gateway).